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Use of NPT reamer

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  • Use of NPT reamer

    Problem: I have an oil filter head for a vintage engine that is tapped to receive an oil pressure gauge that is 1/4 NPT male. Problem is that the female thread is tapped too deep, so the gauge bottoms out before acheiving a seal. I considered using a 1/4 NPT Helicoil, but 1) it's super expensive to insert one coil, and 2) I have no guarantee the result will be any different from what I have now.

    So I'm considering TIGing up the hole, and re-drilling and tapping. That way, I can control the depth of the tapered threads.

    Question: I understand I can tap 1/4 NPT using a 7/16 pilot drill, or a 27/64 pilot drill plus an NPT reamer. 1) What is the advantage, if any, of using a reamer? 2) If using a reamer, how do I know how far to drive the reamer into the pilot hole? I don't want to take a quarter turn too much, and wind up with the same situation I started with.

    Thanks for your advice.

  • #2
    A short, close nipple might seal in the head. Do you have enough room to install a short pipe nipple into the head, then a female coupling, then the gauge?

    I've never had to use a tapered pipe reamer. Always tapped direct into the drilled hole. Takes a bit of torque when nearing thread depth, but after years of practice this became a good reference point for when to stop and check threaded depth.

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    • #3
      If the part is aluminum, there will be no problem just tapping from the base 7/16 hole. Use a good quality tap in good condition. Also, start the tap by hand in a mill or drill press to make sure you get it straight. I sometimes tap this size using a rigid setup in the mill (aluminum).
      Southwest Utah

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      • #4
        The reamer just removes some of the cutting pressure for the tap and the internal thread crest will look nicer. As mentioned above, a lot of people just skip that step in softer metals and it works fine... the reamers usually have a lip the chamfers the top edge and stops you from going too deep. For your engine problem, can you spotface the entry to the thread so the taper isnt as big at the entry anymore?

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        • #5
          Originally posted by mochinist View Post
          The reamer just removes some of the cutting pressure for the tap and the internal thread crest will look nicer. As mentioned above, a lot of people just skip that step in softer metals and it works fine... the reamers usually have a lip the chamfers the top edge and stops you from going too deep. For your engine problem, can you spotface the entry to the thread so the taper isnt as big at the entry anymore?
          That's what I was going to suggest. Spotface the beginning of the hole down so it has more room to screw in deeper. You might need to mill a section away wide enough for the item to turn if a spotface either isn't big enough to clear or you don't have one.

          As regards depth, there are thread gages made that have a flat milled on the threaded gaging surface to tell you where the plane of the workpiece surface should be. But for a one-off hole I would just use the pressure gauge's threads as your gage for depth.
          Last edited by eKretz; 03-20-2021, 06:51 PM.

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          • #6
            Can you not just Tap to 3/8 NPT ?
            Get a 3/8 - 1/4 reducer.
            Last edited by redlee; 03-20-2021, 09:04 PM.
            Beaver County Alberta Canada

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            • #7
              Back in the mid 80s when I worked in the prototype shop at ASCO valves I did a lot of NPT tapping. We had to pull our tooling from the company tool crib and every NPT tapped hole had to be gauged with a standard. That’s all well and good when a hole is deep enough but when you have a shallow bore with a 90 degree passage at the bottom you wind up with a cheater tap to get the right fit. So you bought your own tap and ground the first 5 or 6 threads off. We also kept a little taper pipe plug with a hex in it that we used as a cheater gauge. When the real gauge fit right, we would put the new plug in and surface grind it to within a few thou of the surface. That would serve as a personal gauge when one of the other 23 Tool Makers had the real gauge.

              Not once did we ever have a blueprint that spec’d the use of an NPT reamer.

              Click image for larger version  Name:	35E0B2D8-33C0-4EAD-98B0-6E05ADA53860.jpeg Views:	0 Size:	1.72 MB ID:	1934987
              Last edited by Tim The Grim; 03-20-2021, 09:18 PM. Reason: Darn autocorrect
              Illigitimi non Carborundum 😎
              9X49 Birmingham Mill, Reid Model 2C Grinder, 13x40 ENCO GH Lathe, 6X18 Craftsman lathe, Sherline CNC mill, Eastwood TIG200 AC/DC and lots of stuff from 30+ years in the trade and 15.5 in refinery unit operations. Now retired. El Paso, TX

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              • #8
                Tim, I also have a few modified short projection taps in my tap drawer.
                As you said, very handy for short port work.

                --Doozer
                DZER

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                • #9
                  I have in the past shortened
                  the male end so that the fitting could go in deeper.

                  Jon
                  SW Mi

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                  • #10
                    I've run into similar situations in the past. What I've done is drill and tap the hole over size, next pipe thread size up, even 2 sizes up if needed, make / thread a bushing and secure it in the hole, TIG around it and thread the ID to the proper depth so you get a good seal before the gauge bottoms out and then thread the bushing hole to the proper depth. You can chamfer the hole and the top edge of the bushing so you have a small V groove to weld up.
                    I usually slot the top of the bushing so I can use a screw driver to firmly seat it.
                    I've also had to modify some taps as Tim pictured if I'm working in a blind hole.

                    I wouldn't advise welding up the hole, especially a small dia. one. You usually end up with voids and you won't see them until you drill or thread. That could cause leaks. The weld can also harden and make drilling and tapping difficult.

                    JL...............
                    Last edited by JoeLee; 03-20-2021, 10:22 PM.

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                    • #11
                      Look here, see the little shoulder ground on the reamer at the end of the flutes? You run the reamer in until that shoulder touches off on the surface of the part. As mentioned, you probably won't need a reamer on Aluminum though.

                      https://www.mscdirect.com/product/details/02280030
                      I just need one more tool,just one!

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                      • #12
                        I think I have a pipe tap reamer gage in my stuff. It’s been years since I used it but if you want I would be happy to lend it to you. It is like a taper with marks ground on it for go and no go .
                        [email protected] Hotmail.com
                        Jim

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                        • #13
                          The pressure is low, I have sucessfully got around similar problems by simply wrapping a lot of PTFE tape around the gauge thread.

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                          • #14
                            Ask yourself what advantages may be gained by using a tapered reamer for pipe thread tapping.

                            Less torque is required when tapping a tapered thread into a tapered bore, this is an advantage.
                            A tapered bore does not control the thread length, there is no advantage to be gained this way.

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                            • #15
                              Originally posted by Cheap Jon View Post
                              I have in the past shortened
                              the male end so that the fitting could go in deeper.

                              Jon
                              SW Mi
                              LIKE

                              -Doozer
                              DZER

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